The industrialised way of living produces enormous amounts of solid municipal waste and other forms of solid waste such as for instance rubber tyres, construction materials etc. The vast amounts of these solid wastes have in many highly populated areas grown into a major pollution problem simply due to its volume which has consumed major parts of the available deposition capacity in the area. In addition, there are often strong restrictions to deposition places since major parts of this waste is only slowly biodegradable and do often contain toxic substances.
One very effective way of reducing the volume and weight of solid municipal waste, and which also may destroy many toxic substances, is to burn it in incinerators. This may reduce the volume of uncompacted waste up to 90% leaving an inert residue ash, glass, metal and other solid materials called bottom ash which may be deposited in a landfill. If the combustion process is carefully controlled, the combustible part of the waste will be transformed to mostly CO2, H2O and heat.
Municipal waste is a mixture of many different materials with a wide variety of combustion properties. Thus, in practice there will always be some degree of incomplete combustion involved in solid waste incinerators which produce gaseous by-products such as for instance CO and finely divided particulate material called fly ash. Fly ash includes cinders, dust and soot. In addition there are also difficulties in controlling the temperature in the incinerator so carefully that one has a sufficiently high temperature to achieve an acceptable degree of combustion of the waste, but low enough to avoid the formation of NOx.
In order to avoiding these compounds from reaching the atmosphere, modern incinerators must be equipped with extensive emission-control devices including fabric baghose filters, acid gas scrubbers, electrostatic precipitators etc. These emission-control devices introduces substantial additional costs to the process, and as result, waste incinerators with state of the art emission control are normally up-scaled to capacities of delivering 30-300 MW of heat energy in form of hot water or steam. Such enormous plants require very large amounts of municipal waste (or other fuels) and do also often include very extensive pipelines to deliver the heat energy to numerous customers spread over a wide area. Thus this solution is only suited for major cities and other large heavily populated areas.
For smaller plants, there has presently not been possible to obtain the same degree of emission-control due to the investment and operation costs of the emission-control devices. Presently, this has resulted in more generous emission permits for smaller waste incineration plants which produce less than 30 MW of heat energy and can thus be employed in smaller cities and populated areas.
This is obviously not an environmentally satisfactory solution. The constantly increasing population and energy consumption of the modern society exerts a growing pollution pressure on the environment. One of the most immediate pollution problems in heavy populated areas is the air quality. Due to extensive use of motorised traffic, heating by wood and fossil fuels, industry, etc. the air in heavy populated areas are often locally polluted by small particles of partly or fully unburned carcinogenic remains of fuels such as soot, PAH; acid gases such as NOx, SO2; toxic compounds such as CO, dioxin, ozone, etc. One has recently become aware of that this type of air pollution has a much larger impact on human health than previously assumed, and leads to many common diseases including cancer, auto-immune diseases and respiratory diseases. The latest estimates for Oslo city, population approx. 500000, is that 400 people die each year due to diseases that can be traced to bad air quality, and the frequency of for instance asthma is significantly larger in heavily than in scarcely populated areas. As a result of this knowledge, there are being raised demands for decreasing the emission permits of the above mentioned compounds.
Thus there is a need for waste incinerators that can operate on smaller waste volumes produced by smaller communities and populated areas with the same level of emission-control as the larger incinerators (>30 MW) with full cleansing capacity, and without increasing the price of heat energy. Typical sizes of the smaller plants are in the range of 250 kW to 5 MW
Prior Technology
Most incinerators employs two combustion chambers, a primary combustion chamber where moisture is driven off and the waste is ignited and volatilised, and a second combustion chamber where the remaining unburned gases and particulates are oxidised, eliminating odours and reducing the amount of fly ash in the exhaust. In order to provide enough oxygen for both primary and secondary combustion chambers, air is often supplied and mixed with the burning refuse through openings beneath the grates and/or is admitted to the area from above. There are known solutions where the air stream is maintained by natural draft in chimneys and by mechanical forced-draft fans.
It is well known that the temperature conditions in the combustion zone is the prime factor governing the combustion process. It is vital to obtain a stable and even temperature in the whole combustion zone at a sufficient high level. If the temperature becomes too low, the combustion of the waste will slow down and the degree of incomplete combustion will rise which again increases the levels of unburned remains (CO, PAH, VOC, soot, dioxin etc.) in the exhaust gases, while a too high temperature will increase the amount of NOx. Thus the temperature in the combustion zone should be kept at an even and stable temperature of just below 1200° C.
Despite numerous extensive trials of achieving good control of the air flow in the combustion zones, state of the art incinerators do still produce sufficiently high levels of fly ash and the other above mentioned pollutants that the exhaust must be subject to extensive cleansing by several types of emission-control devices in order to reach environmentally acceptable levels. In addition, most conventional incinerators must also employ expensive pre-treatments of the waste fuel in order to upgrade the fuel and thereby reduce the formation of for instance fly ash.